This invention pertains to a method of controlling an automated welding process, and more particularly, to a method for simultaneously controlling the transverse cross sectional area of the material added during welding, hereinafter the weld bead reinforcement area, and the heat input to the work piece in a gas metal arc (GMA) welding process.
In GMA welding, a consumable wire electrode passes through a copper alloy contact tip. Electric potential applied between the contact tip and the metal to be welded (base metal) results in a current in the wire which supports an arc between the wire end and the base metal. The wire electrode is melted by internal resistive power and heat transferred from the arc. Droplets of molten metal are transferred from the wire to the weld pool of the base metal by a combination of gravitational, Lorentz, surface tension and plasma forces. Heat is transferred to the base metal directly from the arc and also by the molten droplets. Electrode wire, molten droplets, weld pool and solidified well bead behind the weld pool are protected from oxidation by a shielding gas, such as argon or CO.sub.2. GMA welding has been automated by providing means for controlling the rate of filler wire feed and means for controlling the weld speed (the relative motion between the contact tip and the work piece). Generally, control of the process has been limited to certain factors which machine builders have been accustomed to such as the filler wire feed rate, welding speed, current and voltage. These are parameters related to the process. To improve the mechanical and metallurgical properties of the finished weld, it would be advantageous to independently control the parameters related directly to the finished weld, such as, the weld reinforcement area and the heat input to the weld.
The weld reinforcement area is a function of the wire size, the wire feed rate and the welding speed, so the weld reinforcement area may be controlled by controlling these parameters. The desired weld reinforcement area will be dictated, in part, by the geometry of the work piece and may require adjustment during a welding run. Such adjustment may be accomplished by means of direct control of the welding speed and/or the filler wire feed rate. However, changing these parameters results not only in a change in the amount of metal deposited in the weld but also in the amount of heat transferred to the base metal. The heat input to the weld is important since it affects the cooling rate which in turn affects the metallurgical properties of the finished weld.
The heat input to the weld may be controlled by controlling the voltage and current. Again, the geometry of the work piece may dictate desired changes in the heat input during the welding run in order to maintain a uniform cooling rate and thus maintain predicably uniform metallurigcal quality in the weld. Heat input may be adjusted by controlling the welding current but this control is normally achieved by altering the wire feed rate thus the control of heat input is at the expense of adversely affecting the weld reinforcement area.